Motor oil breaks down over time. When it breaks down, it loses its effectiveness and can no longer properly protect your engine.In addition to lubricating an engine’s moving parts, motor oil is designed to carry combustion by-products away from the pistons and cylinders. It is designed to deal with the small amounts of water that form as the engine heats and cools, and to collect the dirt and dust that enter the engine through the air-intake system. It also handles acids that are formed by the reaction between water and other contaminants. Sometimes there are even fuel leaks (fuel dilution) or coolant leaks that get into the oil system.

As a car is driven, the level of contamination in the motor oil constantly increases. The oil filter removes particles as the oil passes through the filter, but over time an oil’s additives are used up and the oil itself can start to degrade (oxidize or thicken). At that point, the oil can no longer do its job and must be changed.

The rate at which contamination and additive depletion occurs depends on many variables. One of these is driving conditions, which vary greatly and have a direct effect on the useful life of the oil. Other factors include the precision of ignition, fuel injection or carburetion adjustments, air cleaner service and the general mechanical condition of the engine.

Oil should be changed before the contamination level reaches the point where engine damage can result. Because it is difficult for the individual motorist to determine when the contamination level is too high, automobile manufacturers provide recommended oil change intervals. These change recommendations vary by model year and manufacturer. Recommended intervals and mileage limits also vary with the type of service under which a car operates. More frequent oil changes are recommended for severe service.

What’s good for the environment is good for your wallet as well.

1. After starting the engine, it is best to drive off immediately. Idling causes pollution and excessive fuel consumption.
2. Depress the accelerator gently to drive away and change to a higher gear in time.
3. Do not approach traffic lights at full speed. If it is clear that you will have to stop, begin slowing down well in advance.
4. Driving at high speed (over 50 mph and especially over 60 mph) uses more fuel and causes more pollution.
5. Avoid drag. Where possible keep all windows closed and ensure that the doors, hood, and trunk are securely fastened.
6. Maintain a sufficient distance between your car and the vehicle ahead. This way there is less need to react suddenly, and you can brake and accelerate more smoothly.
7. On curves, reduce speed in time and accelerate smoothly when you are halfway through a turn. Late, heavy braking before a curve followed by rapid acceleration
   leads to excessive fuel consumption.
8. If you watch the road ahead and anticipate any likely problems, your driving style will become smoother and better controlled.
9. Tires in good condition and at the right pressure improve both safety and
   fuel consumption. Be sure to check your tires regularly.
10. Ensure your car is properly and regularly maintained. Poor engine lubrication, wheel alignment and poorly adjuste brakes can increase fuel consumption.

It’s hard to give a specific time or mileage figure because the life of the filter depends on how much crud it ingests. A filter that lasts 20,000 or even 30,000 miles on a vehicle that’s driven mostly on expressways may last only a
month or two in a rural setting where the vehicle is driven frequently on gravel roads. Changing it annually or every 15,000 miles for preventative maintenance may be a good recommendation for the city driver, but not its country cousin.

Regardless of the mileage or time, a filter should be replaced before it reaches the point
where it creates a significant restriction to airflow. But when exactly that point is reached is subject to opinion.

A slightly dirty filter actually cleans more efficiently than a brand new filter. That’s because the debris trapped by the filter element helps screen out smaller particles that try to get through. But eventually every filter reaches the point where it causes enough of a pressure drop to restrict airflow. Fuel economy, performance and emissions begin to
deteriorate and get progressively worse until the dirty filter is replaced.

Many heavy-duty trucks have a “restriction” meter on the air filter housing that signals when the filter is dirty enough to need replacing. But lacking such a device, the best you can
do is guess.

A cabin filter, often called a dust or pollen filter, is a filter that effectively traps pollutants such as mold spores, pollen, dust, bacteria ,exhaust fumes and diesel soot before they reach the interior of your vehicle. By trapping these pollutants before they enter the vehicle it keeps the interior of your vehicle cleaner and reduces the amount of contaminants entering your lungs. Your cabin filter also needs to be replaced on a regular basis to keep your vehicle’s heating and air cooling system operating at its peak condition, much like the a household air-conditioning system. Although vehicle recommendations vary and much depends on the driving conditions, cabin filters should be replaced every 15K to 20K miles or at least once a year.